How many friends do you have? A rough answer can be predicted by the size of a small, almond-shaped brain structure that is
present in a wide range of vertebrates, scientists report today in Nature Neuroscience.

The researchers studied the amygdala, which is involved in inter-personal functions such as interpreting emotional facial
expressions, reacting to visual threats and trusting strangers. Inter-species comparisons in non-human primates have previously
shown that amygdala volume is associated with troop size, suggesting that the brain region supports skills necessary for a
complex social life1.

On the basis of these past findings, psychologist Lisa Feldman Barrett of Northeastern University in Boston, Massachusetts,
wondered whether a larger amygdala size allows some humans to build a richer social world.

Barrett's team measured the amygdala volume in 58 healthy adults using brain images gathered during magnetic resonance imaging
sessions. To construct social networks, the researchers asked the volunteers how many people they kept in regular contact
with, and how many groups those individuals belonged to.

They found that participants who had bigger and more complex social networks had larger amygdala volumes. This effect did
not depend on the age of the volunteers or their own perceived social support or life satisfaction, suggesting that happiness
is not the underlying causal factor that links the size of this brain structure in an individual to their number of friends2.

"We'd all predict this relationship should be found, but [the authors] did it in a very smart way by ruling out other variables,"
says cognitive neuroscientist Kevin Ochsner of Columbia University in New York City. "That's why I think this paper is going
to end up being a citation classic, because it demonstrates the relationship in a way that gives you confidence that it's
real," he adds.

Brain teaser

But it's still a mystery how the amygdala contributes to social networks. Perhaps the structure's response to faces, emotions
or emotional memories influences whether someone decides to develop and maintain relationships, says Brad Dickerson, a cognitive
neuroscientist at Massachusetts General Hospital in Boston, who helped lead the study.

It's likely that social behaviour relies on a much broader set of brain regions, Dickerson says. In the future, the team will
use functional neuroimaging approaches to determine the relationship between patterns of brain activity in an individual and
the size of social groups to which they belong.

Another important question is whether a big amygdala is a cause or a consequence of having a large social network. "In the
end, it's probably some of both," Ochsner says. "But you first had to establish that the relationship really exists before
you could address those critical questions."